Use AND to narrow a search and retrieve records containing all of the words it separates, e.g. scream AND ghost will only find articles containing both of these words.
Use OR to broaden a search and retrieve records containing any of the words it separates, e.g. scream OR ghost will find articles that contain the word scream only, the word ghost only, or articles containing both of these words.
Use NOT to narrow a search and retrieve records that do not contain the term following it, e.g. scream NOT ghost will find records that contain the word scream, but not the word ghost.